The calibration of the sphere gap voltmeter

Under certain conditions the breakdown of the air gap between equal spheres is very constant and that the sphere gap which has already been suggested as a standard for high voltage measurements has been found to be more reliable than the usual methods of test. Different methods of measuring high voltage are discussed and a new method of reading the maximum value of high voltage circuits is described. The method used in calibrating three sets of spheres in terms of maximum voltage is given. The results of many tests show the breakdown voltage to be independent of wave shape and frequency when expressed in terms of the maximum value of the voltage wave. Direct calibrations are shown for 25-cm., 37.5-cm., and 50-cm. spheres up to a voltage (max. value) of 400 kv. The relation between surface intensity at breakdown and ratio of separation to diameter shows the ultimate strength of the air to vary from 30 to 36.6 kv. per centimeter within the working range. The theoretical proportion between voltage, separation and diameter of spheres did not hold, apparently because of extraneous objects. Calibration curves are shown for the 25-cm. 37.5-cm. and 50-cm. spheres, from low voltage up to the voltage corresponding to a diameter separation. The curves are expressed in effective value of voltage and therefore assume a sine wave shape.